Prescription eyeglass lenses are often supplied to an optical laboratory in the form of a plastic blank. The blank is sized in diameter and thickness to be used in a large variety of eyeglass frames and for a large variety of prescriptions. Therefore, before the eyeglasses can be delivered to the customer, the optical laboratory must grind or generate the base and cross curves onto the back of the lens blank, fine and polish the generated surface of the lens blank to optical clarity, and edge the lens blank to the configuration necessary for the particular eyeglass frame chosen by the customer.
The grinding, polishing, and edging operations are performed on machine tools which may be either manually or numerically controlled. It is necessary to position the lens blank during these processes, and the most common method of holding the lens is with a circular block secured to the front face of the blank. A lead solder may be used because of its low melting point (117.degree. -121.degree. F.), speed in setting up, ease in deblocking, ease in reclaiming the alloy, and solid stability during the process.
While this system is commonly used, it suffers from several drawbacks. Even though the solder has a relatively low melting point, the heat can cause problems with some lens materials. For this reason, the plastic lenses are first covered with a tape prior to blocking. In addition, while the alloy may be reclaimed, that requires additional steps, time, and processing equipment. Furthermore, this alloy system poses environmental problems. The alloy not only contains lead, but also cadmium. Heavy metals may pose health risks to employees handling them and breathing their fumes, as well as a risk to the environment, particularly in the disposal of the water used in the reclaiming tank.
Alternative methods of securing the block to the lens blank, such as with the use of a double-sided adhesive pad, are known. While these adhesive pads overcome of many of the environmental and health concerns, several problems still exist. The appropriate adhesive must exhibit a high resistance to torque because the lens blank is rotated on its geometric axis during the various generating and finishing processes. At the same time, however, the adhesive must have a low resistance to shear, because lab personnel must be able to remove the block and adhesive pad from the finished lens blank. In addition, the adhesive itself must be easily removed with solvents which will not harm the lens blank.
Another more critical problem with an adhesive pad is its susceptibility to deformation during the generating, fining, and polishing processes. During generation, the block is held in a collet and rotated while a cutting tool is brought into contact with the lens blank. To hold the lens on axis, chuck centering pins engage holes in the block. In contrast, during fining and polishing, pins extending from the cylinder machine engage holes in the lens block for moving the generated lens over a lap. The pressure of the tool and the lap against the lens blank may cause relative movement between the lens blank and the block because of the deformable nature of the adhesive pad. This relative motion, particularly when the pad becomes wet, may adversely affect the optics of the lens.